Anodically oxidizable metal powder

ABSTRACT

An anodically oxidizable metal powder for use in electrolytic capacitor applications, the individual metal particles containing in depth and/or surface evenly or unevenly distributed amounts of the metals molybdenum and/or vanadium and/or tungsten and/or hafnium.

Klimt] Katee [.1] tent [1 1 Ronneau et al.

[ 1 ANODICALLY OX1DIZABLE METAL POWDER [75] Inventors: Gerard S.lRonneau, Edegen; Pierre D. Debacker, Hoboken; Hugo L. Carpentier,Aartselaar, all of Belgium [73] Assignee: Metallurgie Hoboken-Overpelt[22] Filed: Feb. 5, 1974 [21] Appl. No.: 439,805

[52] US. Cl. 75/0.5 BB, 29/192 R, 29/192 CP,

75/174 [51] Int. Cl B221 9/00, C22c 27/00 [58] Field of Search 29/192CP, 192 R, 182;

75/05 BB, 05 BA, 174

[4 .1 Feb. 18, 1975 [56] References Cited UNITED STATES PATENTS3,203,793 8/1965 Hand 75/174 3,285,716 11/1966 Contant 75/174 3,415,63912/1968 Daendliker et a1. 75/0.5 BB

Primary Examiner-W. Stallard Attorney, Agent, or Firm-Fred Philpitt [57]ABSTRACT 6 Claims, N0 Drawings I ANODICALLY OXIDIZAIBLE METAL POWDER Thepresent invention relates to improvements in powders used in themanufacture of electrolytic capacitors.

It is well known that electrolytic capacitors have to meet a largenumber of conditions which it is difficult to obtain simultaneously asthey are often contradictory. In the present state of the technique andtaking into account the intrinsic characteristics of the powders used inbuilding up a capacitor, such as tantalum or niobium powders, there issome difficulty in achieving an acceptable compromise amongst thevarious electrical characteristics demanded of a capacitor.

The new powders, object of the present invention, provide the means ofsolving the above problem as will appear from the examples of comparableresults set out in Table I hereafter.

In accordance with the present invention there is provided a metal inpowder form, whose particles are made up of an anodically oxidizablemetal, also recognized as valve metal or metal converting alternatingcurrent to direct current, such as .tantalum or niobium powder,containing molybdenum and/or vanadium and/or tungsten and/or hafnium incontents comprised between 0.06 and 2 per cent on an additive metal totantalum or niobium metal basis, these additive metal contents referringto each additive metal separately or to the additive metals as a whole.

The particles of another powder according to the invention are made upof a nucleus of anodically oxidizable metal covered with a surface layerof the same metal, such layer containing molybdenum and/or vanadiumand/or tungsten and/or hafnium in contents comprised between 0.01 and 2per cent on an additive metal to tantalum or niobium metal basis, theseadditive metal contents referring severally to each additive metalseparately or to the additive metals as a whole. The thickness ofsaidsurface layer will preferentially be less than 3,000 Angstrom.

Still another powder according to the invention is one whose particlesare made up a nucleus of anodically oxidizable metal covered with eithera continuous or a noncontinuous surface layer, or one or more areas onthe surface, made up of molybdenum and/or vanadium and/or tungstenand/of hafnium. The thickness of said surface layer will preferentiallybe less than 3,000 Angstrom.

It has been found that powders according to the invention improve thecharacteristics of the dielectric (electrolytic oxide) built up on thesurface of the particles of which these powders are made up. Suchimproved characteristics give better performance to the capacitormanufactured from these powders. In effect, it has been found on suchcapacitors manufactured from powders according to the invention asizeable decrease of the capacitance variance as a function'of operatingtemperature; in addition, such capacitors withstand reverse voltageshigher than those that can be withstood by capacitors manufactured froma powder that has not been treated according to the present inventionand the performances under life test are better.

The powders according to the present invention can be manufactured by anumber of well known methods such as: co'fusion and embrittlement,sodiothermic reduction, fused electrolysis, hydrogen reduction on afluidized bed.

a. One suitable powder which can be processed in accordance with thepresent invention is a tantalum powder obtained by electron-beam fusionand hydrogen embrittlement. Such high-purity powder is thoroughlyblended with molybdenum powder in proportion of to .2 by weight. Thepowder blend is compacted in an isostatic press by a well-knowntechnique and the resulting bars are melted in an electron beam furnace.The ingot is hydrided to a hydrogen content of about 0.4 per cent byweight, then it is crushed and pulverized and the powder obtained isdegassed and submitted to chemical assay. The desired molybdenum contentis traced back in the powder with more or less 10 per cent accuracy.

b. Another type of powder which can be prepared in accordance with thepresent invention is made by charging a quantity of potassium doublefluoride into a sodiothermic reduction vessel. The reduction occurs at atemperature of about 800C in a fused salt solution containing sodiumchloride: for instance 300 kg of potassium tantalum double fluoride in10 kg'sodium chloride. A stoich-iometric quantity of sodium in liquidform is slowly added to the fuses salt mass; a dendritic tantalum powderis rapidly formed. When the reaction signs (exothermicity) tend todisappear, indicating that the chemical reaction has been completed, aquantity of 260 g of vanadium trichloride is charged into thesodiothermic reduction vessel, then g of sodium are added. Aftercrushing the sponge containing the tantalum, and washing the crushedsponge, a tantalum powder is obtained which contains about 600 parts permillion of vanadium, this latter metal being located on the surface ofthe powder particles. c. Still another type of powder which can beprepared in accordance with the present invention is made by anoperatingprocess similar to the one described just above. The particles of thispowder show a surface layer containing both the matrix metal, which istantalum or niobium, and the doping element. To this end, the reaction,scheduled for reducing 300 kg of potassium tantalum double fluoride isstopped when 90 per cent. of the required quantity of sodium has beenadded; then a reducible compound, as in the process described above, isinjected into the reaction vessel. Then the addition of sodium isresumed and the reduction together of the remaining 10 per cent. of thematrix metal and of the additive element occurs simultaneously on thesurface of the powder particles obtained during the first phase of thereaction.

EXAMPLES.

Table I below records the resultsof measurements made on electrolyticcapacitors manufactured from two powders of the invention; moreexplicitly, one of these powders was made according to example (a)above; it is referenced in table I by the letter (a); it contains onaverage 2,100 parts per million by weight of molybdenum (0.21 per cent).The other powder was made according to example (c); it is referenced intable I by the letter (c); it contains on average 1050 parts per millionby weight of molybdenum (0.105 per cent). For purposes of comparison,table I also includes the resultats of measurements made on capacitorsmanufactured from a powder such as can be found on the market, beingpowder P38 from Reframet-Hoboken. This powder represents the status ofperformance that can presently be achieved without the improvementsbrought by the powders of the present invention; it was manufacturedaccording to the procedure of example (a); nevertheless, contrary to theinvention, it does not contain any traceable amount, added naturally,accidentally or intentionally, of molybdenum or vanadium or tungsten rhafnium that can be determined by chemical analysis.

The electrolytic capacitors manufactured from these three powders are oftype C 40 volts and 50 volts with solid electrolyte (manganese dioxide)and manufactured by a strictly identical process, so as to lend resultsan absolute value for purposes of comparison.

Under Section A of Table 1, characteristics of capacitor anodes will befound such as they can be measured in a liquid electrolyte of phosphoricacid before insertion into the capacitor case and before impregnationwith manganese nitrate, each of these steps being well known by thetrade.

Section B of Table I lists the main characteristics of finishedelectrolytic capacitors, such as capacitance, leakage current anddissipation factor, but the significant parameters of variance ofcapacitance in relation to capacitance at 25C for capacitances measuredat these ratios being expressed in percentage points in Table I. Thesignificant advantage provided by the powders of the invention overanother powder can clearly be seen; there isless capacitance variation.

From Section C it can be seen that the advantage of powders of theinvention is also obvious with respect to leakage current ofelectrolytic capacitors manufactured from these powders as capacitorsbreaking down during life tests are on average fewer.

From Section D it will be noticed that capacitors made from the powdersof the invention withstand higher reverse voltages (with negativepolarization of tantalum) than capacitors made from other powders. Thisadvantage is expressed in the leakage currents and 555C, 85C and 125C.

the breakdown voltages.

TABLE I Performance of type C capacitors Powder of Untreated Powder ofPowder of Untreated Powder of 1 reference the invention the inventionreference the invention the invention Units powder 1050 ppm Mo 2100 ppmMo powder 1050 ppm Mo 2100 ppm Mo on the surface in the matrix on thesurface in the matrix (a) (c) A.- Anode characteristics (average ofmeasurements on 500 anodes) Weight g l 1 1 1 1 1 Diameter mm 4.47 4.494.48 4.47 4.49 4.48 Height mm 7.36 7.32 7.33 7.36 7.32 7.33 Sinteringtemp. C 1850 1850 1850 1850 1850 1850 Sintering time min. 30 30 30 30 30Dissipation 7r 14 14 14 14 14 14 Leakage #Amp/g 0.57 0.80 0.71 0.57 0.800.71 Capacitance uFV/g 3841 3858 3732 3841 3868 3732 B.- Characteristicsof type C capacitors 40 V and V (average of 20 capacitors) C/40 V C/50 VCapacitance ,uF 21.5 22.0 22.2 17.7 17.2 17.2 Leakage uAmp/capacitor0.087 0.141 0.084 0.166 1.08 0.25 Dissipation 72 1.67 0.88 0.86 1.380.86 0.88 A cap(-C) 4.36 2.32 1.84 -3.9 2.3 .1.74 A cap(+C) +5.1 +2.72+2.7 +4.5 +2.9 +2.3 A cap(+125C) 7: +8.8 +5.0 +4.97 +7.90 +4.6 +4.8

' C.- Characteristics under life test (average of 10 capacitors) Testaccording to CCTU O2-12B C/40 V C/SO V 85C 1007(Vn(:) after 2000 hLeakage ,uAmp/capacitor 0.539 0.414 0.393 0.182 0.288 0.192 Breakdowns v0 0 0 0 0 0 after 4000 h Leakage ,uAmp/capacitor 0.747 0.446 0.368 0.4140.300 0.260 Breakdowns 1 .0 0 1 0 0 0 125C-667cVn(:) t

after 2000 h Leakage ,uAmp/capacitor 0.436 0.450 0.1 14' 0.378 0.9660.385 Breakdowns 0 0 0 '1 0 0 after 4000 h Leakage ,uAmp/capacitor 0.8670.623 0.235 0.247 t 0.440 0.26 Breakdowns 2 0 2 1 0' D.- Characteristicsunder direct (positive) and reverse (negative) voltages (average of 5capacitors) C/40 V BDV C/40 V Leakage at POSITIVE POLARIZATION OFTANTALUM 120 V #Amp/capacitor 100 110 105 V V 80 V #Amp/capacitor 0.4 430 V #Amp/capacitor 0.1 0.4 3 V ,uAmp/capacitor 0.01 0.08 NEGATlVEPOLARIZATION OF TANTALUM 10 V uAmp/capacitor 30 0.5 20 V #Amp/capacitor1000 40 25 V 32 V 30 V #Amp/capacitor Breakdown BDV Breakdown voltage(z) Vn Nominal voltage What we claim is:

1. A valve-metal base powder for use in electronics, the grains of whichpowder are made up of the said valve-metal containing between about 0.06and about 2 percent by weight of at least one additive metal chosen inthe group consisting of Mo, V, W, and Hf.

2. A valve-metal base powder for use in electronics, the grains of whichpowder are made up of a nucleus of the said valve-meta] covered with asurface layer of the same valve-metal containing between about 0.01 andabout 2 percent by weight of at least one additive metal chosen in thegroup consisting of No, V, W and Hf.

3. A powder as claimed in claim 2, in which the thickness of the surfacelayer is lower than 3,000 Angstroms.

4. A valve-metal base powder for use in electronics. the grains of whichpowder are made up of a nucleus of the said valve-metal covered with asurface layer consisting of at least one metal chosen in the groupconsisting of Mo, V, W and Hf.

5. A powder as claimed in claim 4, in which the thickness of the surfacelayer is lower than 3,000 Angstroms.

6. A powder as claimed in claim 1, in which the said valvemetal ischosen in the group consisting of tantalum and niobium.

1. A VALVE E-METAL BASE POWDER FOR USE IN ELECTRONICS, THE GRAINS OFWHICH POWDER ARE MADE UP OF THE SAID VALVE-METAL CONTAINING BETWEENABOUT 0.06 AND ABOUT 2 PERCENT BY WEIGHT OF AT LEAST ONE ADDITIVE METALCHOSEN IN THE GROUP CONSISTING OF MO, V, W, AND HF.
 2. A valve-metalbase powder for use in electronics, the grains of which powder are madeup of a nucleus of the said valve-metal covered with a surface layer ofthe same valve-metal containing between about 0.01 and about 2 percentby weight of at least one additive metal chosen in the group consistingof No, V, W and Hf.
 3. A powder as claimed in claim 2, in which thethickness of the surface layer is lower than 3,000 Angstroms.
 4. Avalve-metal base powder for use in electronics, the grains of whichpowder are made up of a nucleus of the said valve-metal covered with asurface layer consisting of at least one metal chosen in the groupconsisting of Mo, V, W and Hf.
 5. A powder as claimed in claim 4, inwhich the thickness of the surface layer is lower than 3,000 Angstroms.6. A powder as claimed in claim 1, in which the said valve-metal ischosen in the group consisting of tantalum and niobium.